Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
Universal Mobile Telecommunications System (UMTS) is one of the third-generation (3G) cell phone technologies. UTRAN, short for UMTS Terrestrial Radio Access Network, is a collective term for the Node-B's and Radio Network Controllers which make up the UMTS radio access network. This communications network can carry many traffic types from real-time Circuit Switched to IP based Packet Switched. The UTRAN allows connectivity between the UE (user equipment) and the core network. The UTRAN contains the base stations, which are called Node Bs, and Radio Network Controllers (RNC). The RNC provides control functionalities for one or more Node Bs. A Node B and an RNC can be the same device, although typical implementations have a separate RNC located in a central office serving multiple Node B's. Despite the fact that they do not have to be physically separated, there is a logical interface between them known as the Iub. The RNC and its corresponding Node Bs are called the Radio Network Subsystem (RNS). There can be more than one RNS present in an UTRAN.
3GPP LTE (Long Term Evolution) is the name given to a project within the Third Generation Partnership Project (3GPP) to improve the UMTS mobile phone standard to cope with future requirements. Goals include improving efficiency, lowering costs, improving services, making use of new spectrum opportunities, and better integration with other open standards. The LTE system is described in the Evolved UTRA (EUTRA) and Evolved UTRAN (EUTRAN) series of specifications.
Semi-persistent scheduling (SPS) is a set of techniques for efficiently assigning resources for periodic traffic in a wireless communication system to support resource assignment with as little overhead as possible in order to improve system capacity.
Current communication system uses a 16-bit cyclic redundancy check (CRC) on all physical downlink control channel (PDCCH) formats. Several formats exist for PDCCH, including downlink (DL), DL compact, UL, and power control. In addition various groupings can be done. As a result the UE must perform about 40 blind decodes per PDCCH per subframe. After decoding is done, UE computes a CRC (X-CRC) over the decoded bits. The X-CRC is checked against the CRC indicated in the PDCCH. If there is a match, the PDCCH is deemed for the UE, its content is interpreted and appropriate actions are performed, i.e. either a transmission or a reception occurs.
A false alarm occurs when the X-CRC matches with the CRC, however, that PDCCH is not meant for that UE. The PDCCH payload can be considered to largely contain random bits in this instance yet the UE performs actions as indicated. Assuming PDCCH bits are random, a false alarm occurs on average every 216 times the CRC check is done. In the worst case the UE performs 40 blind decodes per subframe, and there are 1000 subframes per second. In that case a false alarm will occur for that UE every 216/(40*1000)=1.6 seconds.
When dynamic scheduling is used, the consequence of false alarms is limited to one set of HARQ (Hybrid Automatic-Repeat-Request) transmission and possible subsequent retransmissions. Therefore the consequences are limited.
However when a UE is configured for semi-persistent scheduling (SPS), the consequences can be more serious, especially on the uplink. The evolved base node (eNB) uses a PDCCH with a SPS indication to start SPS. The grant indicated in PDCCH will then repeat every serving interval until revoked. A typical serving interval is 20 ms for VoIP.